Theoretical investigation on the combustion kinetics of nitroalkanes/ammonia: hydrogen abstraction reactions from C1–C5 nitroalkanes by ṄH2 radicals

Nitroalkanes found application as fuel initiators/additives and propellants, being commonly utilized in transportation or propulsion systems. Recently, it has attracted widespread attention in ammonia/reactive fuel co-combustion as a high-performance additive to improve ammonia combustion. Thus, C N interactions play a pivotal role during the nitroalkane/ammonia blending process, especially for the hydrogen atom abstraction reaction by ṄH 2 radicals. The present study thus concentrates on a systematic kinetic investigation of the hydrogen abstraction reactions between C1-C5 nitroalkanes and ṄH 2 radicals. Geometry optimization and frequency calculations, as well as intrinsic reaction coordinate and one-dimensional hindered rotor calculations, were performed at the M06-2X/6-311++G(d,p) level of theory. High-level single-point energies were ultimately calculated at the QCISD(T)/CBS level of theory, incorporating zero-point energies. The high-pressure limit rate constants for 38 reactions of 12 nitroalkanes with ṄH 2 radicals were computed over the temperature range of 500 K to 2000 K utilizing transition state theory. Rate constants for each nitroalkane and the average rate constant rules for ten reaction classes, including abstraction from primary, secondary, and tertiary carbons at the α, β, γ , and δ sites, are provided in this work. Results indicate that the nitro functional group exerts a marginal effect on lowering the α site energy barrier, and the rate constants follow the order: primary < secondary < tertiary. Concurrently, a comprehensive comparison of the average rate constants for analogous structures between nitroalkanes, alkanes, alcohols, and primary amines was performed to elucidate the impact of disparate functional groups on the kinetic parameters. The rate rules exhibited the following trend: nitroalkanes < alkanes < alcohols < primary amines at the α site. The updated kinetic parameters were incorporated into the nitroalkane/NH 3 mechanism to assess their impact on predictions for nitrogen-containing species. These calculated kinetic parameters support a better understanding of the combustion chemistry of nitroalkane/NH 3 blends. Novelty and significance statement Nitroalkanes, particularly nitromethane, have been used as fuel additives to enhance the low-temperature combustion performance of NH 3 /methanol blends. As such, nitromethane serves as an effective NO precursor, facilitating the investigation of high-temperature NH 3 /NO interactions. C N cross-reactions play an important role during the nitroalkane/NH 3 blending process. This work is the first kinetic investigation on the H-atom abstraction reactions between nitroalkanes and amino radicals. A total of 38 reactions were identified, which were subsequently categorized into 10 distinct reaction classes. Thus, rate constant rules for the abstractions of nitroalkanes with ṄH 2 radicals were established, and then compared with those of analogous structures for alkanes, alcohols, and primary amines. This revealed the influence of functional groups on kinetic parameters. The fundamental combustion data presented herein provide critical insights for understanding the combustion chemistry of nitroalkane/NH 3 blends.